规律
    每一段按如下值计算位数，相加即是总的掩码长度

    数值 位
    255 8
    254 7
    252 6
    248 5
    240 4
    224 3
    192 2
    128 1
    0 0

    掩码 位 IP数
    255.255.255.255 32 1
    255.255.255.254 31 2
    255.255.255.252 30 4
    255.255.255.248 29 8
    255.255.255.240 28 16
    255.255.255.224 27 32
    255.255.255.192 26 64
    255.255.255.128 25 128

    255.255.255.0 24 256
    255.255.254.0 23 512
    255.255.252.0 22 1024
    255.255.248.0 21 2048
    255.255.240.0 20 4096
    255.255.224.0 19 8192
    255.255.192.0 18 16384
    255.255.128.0 17 32768

    255.255.0.0 16 65536
    255.254.0.0 15 131072
    255.252.0.0 14 262144
    255.248.0.0 13 524288
    255.240.0.0 12 1048576
    255.224.0.0 11 2097152
    255.192.0.0 10 4194304
    255.128.0.0 9 8388608

    255.0.0.0 8 16777216
    254.0.0.0 7 33554432
    252.0.0.0 6 67108864
    248.0.0.0 5 134217728
    240.0.0.0 4 268435456
    224.0.0.0 3 536870912
    192.0.0.0 2 1073741824
    128.0.0.0 1 2147483648
    0.0.0.0 0 4294967296